The present invention relates to the formation of structures of more than one alloy composition. More particularly, it relates to a method by which disks can be formed having an inner core alloy of one composition and a rim alloy of a different composition.
It is known that superalloys including nickel base and iron base superalloys have been employed extensively in applications which require high strength at high temperature. The design of jet engines has in large part been determined by the properties which superalloys used as fabricating materials for components of the engine can display. As the properties of the alloys are improved the design of the jet engine improves and greater thrust to weight ratios are achieved. Generally, higher temperature operation results in greater fuel efficiency for such engines and the drive for higher operating temperatures and for superalloy materials which can operate at such higher temperatures is a continuous design criteria for fabrication of more and more efficient jet engines. The need for higher temperature capability in high strength superalloys continues as efforts are made to continue to improve operating performance for jet engines.
Many metallurgical advances have assisted in improving high strength superalloys. These have included the increase in the precipitate volume fraction for the gamma prime precipitate strengthening agent of such alloys. Also improvements have been made through powder metallurgy and through the use of isothermal forging. Improvements in the alloy temperature capability of superalloys have been achieved in this way. It has also been recognized that not all components of a jet engine are subject to the same operating conditions and that different metallurgical compositions may be employed in different components of the engine to best suit the needs of that component.
There are some parts where tradeoffs have been made in properties because the part is large enough so that the engine operating conditions over the full extent of the part are not uniform. In other words, certain large pieces which are installed in an engine encounter different temperatures and different property requirements and service from one portion of the component to another. Accordingly, for such large components it is necessary to sacrifice a property in one location of the component in order to obtain a acceptable property at another location. Such different properties are needed for example in engine disks which rotate at high speeds of 12,000 revolutions per minute and more and result in the application of high stress to portions of the disk and particularly to the outer portions of the disk.
In order to compensate for the different property requirements of the different portions of the disk, schemes and methods have been devised to impart desirable combinations of properties to the inner and outer portions of such disks. For example, the U.S. Pat. No. 4,820,358 issued to the assignee of the subject application has taught a method by which a disk made of a single alloy can be given different properties at its inner or core portion as contrasted with its outer or rim portion. The attainment of different properties in the different portions of the disk is a valuable achievement.
Other efforts have been made to form an inner portion of a disk of one alloy and an outer portion of a different alloy. However, problems have arisen where efforts are made to join the two alloys together. If oxide layers exist at the boundary it is difficult to be sure that any welding that has occurred overcomes the presence of the oxide and does not leave a region of weakness in the disk. The detection of flaws in such weldments between an inner and outer portion of a disk is difficult.
The present method is directed toward overcoming the difficulty of having an oxide layer which can cause points of weakness or imperfect welds between the inner and outer portions of alloy disks where such inner and outer portions are of different alloy materials.